-LRB- CNN -RRB- -- The airline industry 's latest tactic to increase revenue -- charging premiums for window and aisle seats -- is making it hard for groups to fly together , say passenger advocates .

George Hobica , founder of travel site AirfareWatchdog , said his organization had received a spate of complaints from Americans who had struck problems being seated together on flights . Complainants included a woman who had been unable to sit next to her husband , who had a medical condition .

He said the problems were arising as airlines set aside more desirable window and aisle seats for passengers who pay a premium , leaving those who do not pay with center-row seats , separated from other members of their party unless they have paid the surcharge .

`` The airlines are basically trying to squeeze out more fees in the face of higher costs , and it is inconveniencing a lot of passengers , '' said Hobica .

The charges for window or aisle seats in the front half of the plane , costing as much as $ 29 each way on U.S. domestic flights and $ 59 on international flights , are the latest of a raft of passenger fees introduced in recent years as the airline industry looks to generate new streams of revenue .

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The issue has arisen in the United States , where Delta , American Airlines , and low-cost carriers US Airways , Frontier , Spirit and Allegiant have implemented charges for `` preferred seating . '' Delta spokeswoman Katie Hulme said the move reflected the fact that some seats were more desirable than others , and that it gave passengers greater choice and flexibility in where they were seated .

`` Offering preferred seats for sale to all Delta passengers means that we are offering different seat selection options to enable more passengers to travel in their seat of choice , '' she said .

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The notion of paying extra to reserve more desirable seating is catching on more widely across the industry . Across the Atlantic , low-cost Ryanair rolled out reservations for certain seats across all its routes in January , while its rival Easyjet has also been experimenting with the practice .

The implementation of end-of-row seat fees is part of a broader airline trend to charge for amenities previously included in the standard fare , such as movies , water , pillows and blankets .

In response to soaring fuel prices four years ago , airlines also began introducing charges for checked baggage , generating huge sums . But last year the revenue from checked bags for U.S. airlines fell for the first time , suggesting passengers were trying to avoid fees by packing light when they traveled .

Two U.S. airlines , Spirit and Allegiant , also charge passengers for carry-on baggage .

Hobica said part of the airlines ' rationale was `` to cater to the people that have paid the most for their seats . ''

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Kate Hanni , of passenger rights organization FlyersRights , said the aisle and window surcharges made air travel an even more daunting prospect for families .

`` Families have already been having a hard time , so this is the icing on the cake , '' she said . `` Can you imagine getting on a plane and not being seated next to your three-year-old ? ''

While gate staff would typically try to seat families together , removing aisle and window seats as options was making this impossible , said Hanni .

`` The predictability of air travel has really gone away . This is one more layer where people ca n't predict what 's going to happen -- it 's a crap shoot , '' she said .

`` They 're holding the aisle and window seats out for higher prices -- and trust me , this is just a revenue grab . ''

Hobica said while the new price structure was causing problems , he could understand the logic behind it . `` When you buy a ticket to a Broadway show , you do pay more for better seating , '' he said . `` If passengers were willing to pay higher fares at the beginning , '' he said , the problems could be avoided .

Hulme said that the number of rows in which window and aisle seats were set aside as `` preferred seating '' varied from aircraft to aircraft , but did not extend through the entire plane . Many of those who traveled in groups were leisure travelers who tended to book their fares in advance , and should encounter few difficulties being seated together , she said .

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A number of U.S. airlines now charge extra for passengers to reserve a window or aisle seat

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Passengers reporting difficulties traveling in groups as end-of-row seats are already booked

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Fees are being introduced as airlines try to generate extra streams of revenue